The Viper blocks are incremental upgrades as time went. Block 50/52 is known to be the most capable but the heaviest in US service. After Block 25, the last digit represents the engine (0=GE with a big air intake, or « big mouth » and 2=PW). The first digit represents the actual avionics upgrade block. Block 50/52 were known as CJ and those that were upgraded are now called CM. Blocks 60s/70s are export variants. The other blocks are one ofs or concepts.
As for the black fighter project, this is hardly a new thing. Many aircraft (some that saw operational actions, some remained developmental platforms) were developed that way. Generally speaking, non-export platforms are developed under a heavier veil of secrecy than export platforms.